Rinse beans and cut into pieces, if desired. Cook in boiling salted water for 8-10 minutes. Drain and rinse in cold water, drain well.
Rinse, trim and slice radishes. Rinse and spin dry watercress. Rinse and pat dry tomatoes, cut into wedges. Peel and dice onion.
Whisk vinegar with walnut oil and season with salt, pepper and sugar. Combine all salad ingredients and toss with the dressing. Arrange salad on plates and serve.